A hydraulic circuit for a construction machine having a floating function in accordance with the prior art is disclosed in Korean Patent Registration No. 10-0621977. As shown in FIG. 1, the hydraulic circuit for a construction machine having a floating function includes:
at least two hydraulic pumps 1 and 2;
a hydraulic cylinder 3 that is driven by hydraulic fluids supplied from the hydraulic pumps 1 and 2;
a boom driving control valve 4 that is installed in a flow path between any one 1 of the hydraulic pumps 1 and 2 and the hydraulic cylinder 3 and is configured to be shifted to control a start, a stop, and a direction change of the hydraulic cylinder 3;
a boom confluence control valve 5 that is installed in a flow path between the other 2 of the hydraulic pumps 1 and 2 and the hydraulic cylinder 3 and is configured to be shifted to allow the hydraulic fluid discharged from the hydraulic pump 2 to join the hydraulic fluid that has passed through the boom driving control valve 4 to cause the joined hydraulic fluids to be supplied to a large chamber of the hydraulic cylinder 3, or to allow hydraulic fluids of the large chamber and a small chamber of the hydraulic cylinder 3 to join together so as to be supplied to a hydraulic tank 6 to shift the boom confluence control valve 5 to a floating state; and
a control valve 7 that is installed in a flow path between a manipulation lever (not shown), and the boom driving control valve 4 and the boom confluence control valve 5, and configured to be shifted to supply the hydraulic fluid discharged from the hydraulic pump 1 to the small chamber of the hydraulic cylinder 3 through application of the boom-down pilot pressure to the boom driving control valve 4, or to shift the boom confluence control valve 5 to an on state to cause the boom confluence control valve 5 be shifted to the floating state through application of the boom-down pilot pressure to the boom confluence control valve 5.
When a spool of the control valve 7 is shifted to the left on the drawing sheet in response to an electrical signal applied thereto, a boom-down pilot pressure is applied to one end of the boom confluence control valve 5 via the control valve 7 by the manipulation of the manipulation lever to cause a spool of the boom confluence control valve 5 to be shifted to the left on the drawing sheet.
In other words, the boom confluence control valve 5 is shifted to the floating state. The boom confluence control valve 5 is shifted to allow the hydraulic fluids of the large chamber and the small chamber of the hydraulic cylinder 3 to join together in the boom confluence control valve 5 so as to be returned to the hydraulic fluid tank 6 so that the boom confluence control valve 5 is shifted to the floating state.
As described above, when the boom confluence control valve 5 is shifted to the floating state by the shift of the control valve 7, the boom-down pilot pressure is not applied to the boom driving control valve 4, and thus the hydraulic fluid from the hydraulic pump 1 is not supplied to the small chamber of the hydraulic cylinder 3. As a result, the boom cannot descend in a state where the control valve 7 is switched to the on state, thus making it impossible to perform the jack-up operation.